This looks like a parsing issue where Railpack is stripping the bracket syntax from your install command. Since you've already tried single quotes without success, there are a couple of workarounds.

The cleanest approach is to put the install specification in a requirements.txt file instead. Add a line with just .[workers] (or -e .[workers] if you need editable mode) and Railpack will handle it during the automatic dependency installation step without you needing to override the install command at all.

If you need more control, you could use a Dockerfile where shell escaping behaves predictably—something like RUN pip install ".[workers]" will work as expected.

This does seem like a bug in Railpack's command parsing. If the requirements.txt workaround doesn't fit your workflow, it's worth opening an issue at github.com/railwayapp/railpack with the details you've shared here—the fact that it displays correctly in the steps output but executes differently is useful debugging info.